Clinicians and researchers have recognized the value of a completely noninvasive method for measurement of hematocrit or total hemoglobin concentration. U.S. Pat. No. 6,606,509 B2 discloses a device and method for measurement of hematocrit (Hct) by noninvasive means. The changes in the intensities of light of multiple wavelengths transmitted through or reflected light from the tissue location are recorded immediately before and after occluding the flow of venous blood from the tissue location with an occlusion device positioned near the tissue location. As the venous return stops and the incoming arterial blood expands the blood vessels, the light intensities measured within a particular band of near-infrared wavelengths decrease in proportion to the volume of hemoglobin in the tissue location; those intensities measured within a separate band of wavelengths in which water absorbs respond to the difference between the water fractions within the blood and the displaced tissue volume. A mathematical algorithm applied to the time-varying intensities yields a quantitative estimate of the absolute concentration of hemoglobin in the blood. To compensate for the effect of the unknown fraction of water in the extravascular tissue on the Hct measurement, the tissue water fraction is determined before the occlusion cycle begins by measuring the diffuse transmittance or reflectance spectra of the tissue at selected wavelengths.
US 2010/0331636 A1 discloses a method for the non-invasive determination of the concentration of blood constituents, in which a radiation source emits several radiation beams, each with a different wavelength. A first photo detector receives the measurement radiation of each wavelength that is reflected by a body part to be examined. A second photo detector receives the measurement radiation of each wavelength that is transmitted by the body part to be examined. The measurement radiation of each wavelength that is absorbed by the body part to be examined is then determined on the basis of the measurement of the reflected radiation by the first radiation receiver and the measurement of the transmitted radiation by the second radiation receiver. The concentration of the different constituents is calculated from the absorption of the measurement radiation that has been determined for each wavelength.
U.S. Pat. No. 6,611,320 B1 discloses a method for detecting blood characteristics including hemoglobin in a fluid medium using both transmission and reflection of a light beam which forms a quotient.
WO 93/13706 A2 discloses an apparatus and method of measuring blood hematocrit which involves directing first and second wavelengths of light through a blood sample, determining the ratio between pulsatile and non-pulsatile diffuse transmittances measured at each of the first and second wavelengths of light from the blood sample, and determining blood hematocrit of the blood sample from the ratio between pulsatile and non-pulsatile diffuse transmittances measured at each of the first and second wavelengths of light from the blood sample.